1. Field of the Invention
This invention relates to an optical-disk recording method, recording apparatus and reproducing apparatus using an optical disk recording medium having a plurality of servo areas circumferentially provided at a predetermined interval and holographic recording areas provided between the servo areas, and more particularly to an optical-disk recording method, recording apparatus and reproducing apparatus for continuously recording/reproducing information with ultra-high density by the utilization of holography.
2. Description of the Related Art
There is known a holographic recording scheme for recording, by hologram, information with ultra-high density onto an optical disk recording medium. In the holographic recording scheme, an interference fringe pattern is generated within the optical disk recording medium by superposing together an information light carrying image information and a recording reference light. By recording the interference fringe pattern in the optical disk recording medium, image information can be written. For reproducing information from the recorded interference fringe pattern, a reproducing reference light similar to the light of upon writing is irradiated to the interference infringe pattern recorded within the optical disk recording medium to cause diffraction due to an interference fringe pattern, thereby reproducing image information.
Recently, attentions are drawn to the development of volume holography having a recording density further increased by writing, in a three-dimensional fashion, an interference fringe pattern through utilization of a thickness direction of recording layer of the optical disk recording medium. The capacity of information recording can be drastically increased further by the multiplex recording with using such volume holography recording scheme.
International publication No. WO99/44195 discloses an apparatus and method for recording/reproducing information to/from an optical disk recording medium by such volume holography. In order to understand the present invention, explanation will be briefly made on the structure of a recording/reproducing apparatus utilizing volume holography described in that publication. As shown in FIG. 19, an optical disk recording medium 101 is structured with a holographic recording layer 101c provided between circular transparent substrates 101a, 101b, and a reflection film 111d formed on a surface of the transparent substrate 101b opposite to the recording layer 101c, wherein those are bonded on a substrate 10e. In the reflection film 101d, address servo areas are arranged plurally in radial direction of the optical disk recording medium 101 at a predetermined angular interval. Between the address servo areas arranged circumferentially, information recording areas are provided. The address servo area is previously recorded, by embossed pits, with information for focus servo control and tracking servo control as well as address information about information recording area. The information for tracking servo control can use, for example, wobble pits.
The optical disk recording medium concretely has a structure that the transparent substrate 101a, 101b has a suitable thickness, for example, of 0.6 mm or smaller and the holographic recording layer 101c has a suitable thickness, for example, of 10 μm or greater. The holographic recording layer 101c is formed of a holographic recording material which, when irradiated with a laser light for a predetermined time, optical characteristics of refractive index, dielectric constant, reflectivity and the like change depending upon a laser-light intensity. This uses, for example, Photopolymers HRF-600 (product name) by Dupont, or the like.
Recording to the holographic recording layer by volume holography, in one example, is as shown in the figure. Namely, an information light 111 carrying recording information and a recording reference light 112 are simultaneously irradiated at a side of the transparent substrate side 101a to produce an interference fringe in the thickness direction in the holographic recording layer 101c. Such irradiation is continued for a predetermined time to fix the interference fringe pattern in the holographic recording layer 101c, thereby recording information as a stereoscopic hologram.
In the meanwhile, in order to realize high-density recording using a general recording apparatus for optically recording information onto a disk-shaped optical disk recording medium, density increase can be achieved by narrowing the track pitch in information recording area of the optical disk recording medium. In holographic recording, however, the area of a holographic interference region (holographic recording spot) (e.g. φ500 μm) in holographic recording layer is greater compared to the track pitch (e.g. 0.8 μm). For density increase, there is a need for multiplex recording as stated before. It is a usual practice, as shown in FIG. 19, to record holographic recording spots onto the optical disk recording medium 101 by shifting the spots in the horizontal direction for each spot to have a partial overlap with the adjacent spot. In the multiplex recording like this, provided that the diameter of a holographic recording spot is D and the pitch between holographic recording spots is P, multiplex number m is given m=D/P.
However, in the multiplex recording, diffraction efficiency (reproduced light intensity) lowers in proportion to a square of the multiplex number m of hologram. Thus, there is a need to previously determine an optimal multiplex number with taking into account an improved recording density and secured reproduced light intensity, thereby determining based thereon a holographic recording spot pitch to the recording layer. Also in this case, the determined holographic recording spot pitch (P) itself (e.g. 20 μm) is greater relative to the track pitch in the information recording area of the optical disk recording medium. Consequently, in case holograms are to be successively written directly at the determined pitch P of holographic recording spots to the holographic recording layer 101c, recording is possibly made in such a burst fashion that recording is successful on a certain track but impossible on another track. There is a difficulty in continuously reproducing the hologram recorded in such a burst fashion as this.
It is an object of the present invention to provide an optical-disk recording method, recording apparatus and reproducing apparatus which are capable of continuously recording/reproducing a hologram with ultra-high density.